Carbon-fiber-reinforced plastics have excellent specific strength and specific rigidity and thus are useful. Their applications have been widely expanded to aircraft structural members, wind mill blades, and automobile outer panels, as well as computer applications including IC trays, laptop computer housings, and the like, and the demand is increasing year by year. In particular, carbon-fiber-reinforced plastics are light in weight and have excellent strength and stiffness, and thus have been widely used in the aircraft industry, as typified by commercial aircrafts. In recent years, they are also used for large-sized structural members, such as main wings and bodies.
For such a structural member, a cured prepreg laminate, which has particularly excellent dynamic characteristics among fiber-reinforced plastics, is often used. In a fiber-reinforced plastic composed of laid-up prepregs, fibers are unidirectionally aligned, and the fiber volume content is improved, whereby the high fiber elastic modulus and strength of carbon fibers can be best utilized. In addition, when the prepregs are impregnated with a high-performance resin while reducing variation in areal weight, the obtained fiber-reinforced plastic has stable quality. As a weak point of such a fiber-reinforced plastic composed of laid-up prepregs, there has been a problem in that even when interlayer cracking has occurred in the prepregs upon the impact of a foreign substance from out of the plane, and there is delamination inside, the presence of damage cannot be seen from the outside. The presence of delamination leads to a decrease in the compression strength of the structural member. Therefore, for the purpose of ensuring safety during aircraft flight, compressive strength after impact, which is called CAI, has been used as a structural design index. Then, according to Patent Document 1, a thermoplastic resin is formed into fine particles and localized on the surface of a prepreg. As a result, when such prepregs are laid up to form a fiber-reinforced plastic, the thermoplastic resin is accumulated between layers, thereby enhancing the delamination strength. Accordingly, the area of delamination upon the application of out-of-plane impact is reduced, whereby CAI is successfully improved. Currently, such “increased-interlayer-toughness” prepregs have been mainly applied to fiber-reinforced plastics used for primary structural members of aircrafts.
It is known that of the steps for producing a structural member, the shaping step, in which prepregs are made conform to a three dimensional shape and formed into a preform before the forming/curing step using an autoclave or the like, is an important step that influences the success or failure in material quality. When prepregs are shaped layer by layer, a high-quality preform can be obtained, but such a process is high cost and also takes a long period of time. Then, in order to enhance the production efficiency, a shaping method called hot-forming, in which prepregs are previously laid up in planar form into a prepreg laminate at high speed using an automatic machine, and then the prepreg laminate is shaped into a three dimensional shape while applying heat thereto, is used. Patent Document 2 discloses a shaping method in which a prepreg laminate is disposed between a mandrel and an expandable bladder, and the bladder is expanded, thereby pressing the laminate against the mandrel while bending the same.